Liner hangers have long been used in oil and gas recovery operations for suspending or hanging a liner from a well casing. As used herein, the term "liner" means a section of tubing, casing, or similar tubular material to be secured to a larger-diameter downhole tubular generally fixed within the well bore. Included in this definition is a "tieback liner", which is a section of tuving extending upward within the well casing from the hanger, and a "scab liner", which is typically used to repair damaged casing.
A liner normally does not extend to the surface, and is a simple yet highly versatile tubular generally utilized as a cost effective solution to various anticipated or unanticipated downhole problems. Liners may be utilized, for example, to prevent loss of circulation in weak upper zones while drilling with weighted mud to control deeper pressurized zone. Scab liners are frequently used to repair corroded or damaged casing either above or below the liner hanger to allow for continued cost-effective production operations. Liners may also be used to economically conduct cased hole tests of questionable zones, since liners may be "run in" a well much faster than full diameter casing, thereby reducing "trip" time and rig expense. Liners often extend down past the well casing several hundred feet or more into "open hole", and may either be cemented in place or remain supported, solely by the liner hanger.
Mechanically or hydraulically set slips are typically used to effectively interconnect the liner hanger to the casing, and various techniques have been devised for securing a liner to the liner hanger. A fixed interconnection of the liner and the liner hanger is often more difficult to obtain than the casing/liner hanger interconnection, however, and accordingly many prior art liner hangers are intended to cooperate with specially prepared liners. In some instances field welding is used to interconnect the liner with liner hanger components. Other liner hangers require the liners to be threaded with special or "premium" threads, thereby increasing costs and reducing versatility of the liner.
Certain types of liner hangers, such as the Brown Flex Lock liner hanger, does not require special preparation of the liner. These hangers utilize an outer cone sleeve and an inner split-ring locking sleeve with mating threads. Right-hand and left-hand interior threads on the inner locking sleeve bite into the outer surface of the liner as the cone sleeve and a jamb nut are threaded together, thereby causing the locking sleeve to bite the liner. This type of liner hanger allows a customer's standard liner or pipe to be suspended from a casing without modification. The desired axial position of the liner with respect to the hanger can thus be readily adjusted at the well site, and thus this type of Brown liner hanger is accordingly preferred by some customers.
The above described Brown liner hangers are, however, frequently not employed when utilizing hard grades of liners. The "teeth" forming the right-hand and left-hand threads on the inner surface of the locking sleeve are designed to bite into the liner as the outer cone sleeve and a jamb nut are torqued together, but the desired bite has heretofore been difficult to obtain in hard grades of steel liners. Since inadvertent downhole separation of the liner and liner hanger must be avoided to prevent an expensive workover operation, customers often require the more expensive and less versatile liners and hangers when utilizing hard grades of liners.
Threads having a straight buttress thread profile have been provided for mating engagement between the cone sleeve and the inner locking sleeve of the above described Brown liner hangers. While at the surface, the torqued engagement of the cone sleeve and the jamb nut thus provides an axial force which causes the threads on the locking sleeve to slide along the corresponding taper of the thread profile on the cone sleeve, thereby driving the inner teeth on the locking sleeve to bite the liner. When utilizing harder grades of liners, operators may question whether the desired tooth penetration of the locking sleeve to the liner will be obtained to prevent slippage of the liner along the liner hanger as it is lowered into the well. Accordingly, use of the above-described Brown liner hangers has been limited.
If an axially directed load is applied to the Flex Lock liner after it is positioned in the well, a slight additional axial movement between the cone sleeve and the locking sleeve may occur as the locking sleeve continues to slide along the thread profile of the cone sleeve thereby driving the teeth of the locking sleeve into deeper engagement with the liner. This motion is, however, unrestricted since the radial biting force applied by the locking sleeve to the liner may continually increase with an increase in the axial load. Moreover, this continued sliding motion along the taper of the thread profiles results in less threaded engagement between the cone sleeve and the locking sleeve, thereby increasing stress on those components, which may cause failure. Finally, this motion may cause the tapered surfaces of threads on the cone sleeve and locking sleeve to pass completely past each other or "jump" to the next thread, which will then likely continue in rapid fashion until the locking sleeve and cone sleeve separate or fail due to increased stress, again resulting in an expensive workover operation.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for interconnecting a downhole casing with a liner.